1. Field of the Invention
The present invention relates generally to variable-gain amplifiers.
2. Description of the Related Art
Variable-gain amplifiers (VGAs) are especially useful for precise control of signal levels in a wide variety of communication systems (e.g., in transmitters and receivers of mobile handsets). Although a large variety of VGA structures have been proposed, they generally fall in either a closed-loop or an open-loop category.
Realizing VGAs with feedback signals in a closed-loop configuration typically enhances gain accuracy (e.g., gain over temperature and fabrication process variations) at the expense of operational bandwidth. In contrast, realizing VGAs without feedback (i.e., in an open-loop configuration) has typically enhanced operational bandwidth but degraded gain accuracy.
Open-loop VGAs, for example, have been configured with input attenuators (e.g., resistive networks) that are succeeded by wideband, fixed-gain amplifiers. Typically, transistor or diode switches permit signals from successive portions of the input attenuator to be selectively coupled to the succeeding amplifier which provides the VGA""s output signal. These arrangements, however, degrade the VGA""s signal-to-noise performance because they attenuate the input signal relative to the inherent noise that resides at the amplifier""s input.
Accordingly, a need exists for wide-band, low-noise VGAs that can also provide excellent gain accuracy over variations in temperature and fabrication processes
The present invention is directed to variable-gain amplifiers that provide gain accuracy (e.g., over variations in temperature and fabrication processes) while also providing this accuracy over a wide bandwidth and without the signal-to-noise degradation typically associated with signal attentuating elements.
These goals are realized with differential signal and gain amplifiers that include current sources which are controlled by a common error signal Serr. The gain amplifier is supplemented by feedback structure that generates the error signal Serr and controls the amplifier""s transconductance to be the ratio of at least one of currents and resistors. Because such ratios can be well matched (especially in integrated circuit realizations of the variable-gain amplifiers) and because the current source of the signal amplifier is also controlled by the error signal Serr, this wide-band, low-noise open-loop amplifier""s gain is accurately controlled.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.